This invention relates to ion generation, and more particularly, to the compensation for stroke width variations in ion generation imaging.
There are a number of techniques for generating ions to form electrostatic images. These include air gap breakdown, corona discharges, and spark discharges.
Particularly suitable for electrostatic imaging is a device disclosed in Fotland et al. U.S. Pat. 4,155,093. This patent discloses an ion generating device with a solid dielectric member contacted on opposite sides by two electrodes. One of the electrodes contains apertures or edge surfaces, located opposite the other electrode. A varying high voltage applied between the two electrodes generates a pool of positive and negative ions in the apertures. These ions may be extracted by a potential applied between the apertured electrode and a counterelectrode. This apparatus is suitable for electrostatic imaging in that apertures may be configured in a desired shape to create an electrostatic image of corresponding shape. A multiplexible imaging device may be created by patterning an array of opposing electrodes in the matrix crossover arrangement disclosed in Fotland et al. U.S. Pat. 4,267,556. Ions are extracted to form a latent electrostatic pattern of images on an adjacent dielectric member, normally consisting of discrete dots. These discrete images or dots overlap to form particular letters, characters, or symbols. A lesser constant potential may be applied to a third electrode, known disclosed in Carrish U.S. Pat. 4,160,257. The screen electrode is used to counteract the tendency of an electrostatic image of given polarity to attract oppositely charged ions from the discharge aperture when the direct current potential is removed between the control electrode and the conducting sublayer. It was found that the screen electrode provides control over image size by varying the size of the screen aperture, or by varying the voltage applied to the screen electrode, or by varying the distance between the screen electrode aperture and the dielectric cylinder. A reduction in the screen aperture size, for without any compromise in the image charge. This control over image size did not resolve the problem of variations in image size between two or more adjacent images. The failure to independently control individual image size has limited the printing capability of the system.
The stroke width, or the width of the printed image created by the electrostatic discharge from the cartridge, must fall within a certain range of values in order to obtain the desired visual qualities. As the human eye is sensitive to abrupt changes in stroke width, such changes will detract from the visual uniformity and sharpness necessary for many printing applications. The geometry of, or the potential applied to, each aperture of the screen electrode can be varied to compensate for any known change in stroke width. Such changes in stroke width can occur in several ways, for example, realignment of the cartridge with respect to the cylinder. The present invention is designed to compensate for stroke width variations resulting from changes in the distance between the discharge electrode and the dielectric surface on which ions are deposited over the areal extent of the array of electrodes. In practice it was found that this change in distance, caused by the cylindrical shape of the dielectric cylinder and/or reorientation of the cartridge with respect to the cylinder, was the source of these fringe effects. Such reoreintation or skewing of the cartridge is necessary to adjust for the realignment of the printed image caused by the skewing of the image and transfer cylinders. As disclosed in Ser. No. 180,218, the skewing of these cylinders is used to increase the toner transfer efficiency.
Accordingly, it is the principal object of the invention to maintain control over the individual size of electrostatic images generated on the dielectric receptive surface.
A further object of the invention is to enhance the print quality by maintaining uniformly sized electrostatic images that are generated on the dielectric receptive surface.